New cables for GPU makes PC unable to boot

[Flouemz]

I bought a Cooler Master Silent Pro M2 620W PSU some time ago, but unfortunately, it came with 2x 6+2 pin + 8 pin PCIe cables. My GPU (an HD 7870 Vortex II) uses 2 x 6 pin connectors, so I contacted Cooler Master and had them send 2 new PSU cables (2 x 6+2 pin) to me, after I finally decided that I wanted to do some "modding" with my pc (have it cleaned + new fans and lighting)
I plugged in one of the new cables (instead of having to use 2 of the old ones) and tried to boot it up, but my PC did nothing except making a small "click"-sound. I then went back to the old cables and it worked just fine. Can anyone tell me how to fix this, or at least tell me what the problem is? I've read some threads about people not being able to boot with a new PSU or GPU, but is this the same problem? There's no new hardware except a cable involved.
All PC specs:
CPU: FX 8320
MOBO: ASUS Sabertooth 990FX R2.0 with newest BIOS update available
GPU: HD 7870 Vortex II
Boot drive: 120gb SSD (Kingston)
PSU: Cooler Master Silent Pro M2 620W

 

[Eduello]

What if you use the other new cable?

 

[Flouemz]

Doesn't work either. I even had them send me two more new cables, just in case the first new ones were defect. New ones doesn't work either. Tried all of them, even with 2 of them used (1 6-pin from each).

 

[Eduello]

But the old cables still work? That's odd... The PSU has a single 12V rail so that can't be the issue either. The only thing I could think of is that both sets of the new cables happen to be from a bad batch.